A photodetector is a device for receiving light from a target space, generating electric charges corresponding to the received light amount, and taking out the electric charges as a received light output. To obtain a wide dynamic range in this kind of device, it has been proposed to remove, as undesired electric charges, a constant amount of electric charges in a charge transmission channel of a CCD device, and use the remaining electric charges other than the undesired electric charges as effective electric charges for the received light output.
For example, Japanese Patent Early Publication No. 7-22436 or 7-22437 discloses an electric charge transfer device capable of expanding a dynamic range, while keeping the linearity of voltage-electric charge conversion characteristics. This device is provided with a voltage-electric charge conversion means for converting an input voltage into a corresponding amount of electric charges at every cycle of a periodical pulse signal, a plurality of accumulation regions for accumulating the converted electric charges, a charge transfer means for transferring the electric charges from one of the accumulation regions to an adjacent accumulation region. This device also has a first storage region for accumulating a constant amount of electric charges, and a second storage region for accumulating electric charges overflowing from the first storage region, and transferring the electric charges to the charge transfer means. The first storage region has a potential well formed adjacent to the one of the accumulation regions, and a potential barrier provided between the second storage region and the potential well.
According to this device, the constant amount of electric charges in the electric charges converted at every cycle of the periodical pulse signal by the voltage-electric charge conversion means is separated by use of the potential well and the potential barrier, and then returned to the voltage-electric charge conversion means. At the same time, the remaining electric charges in the electric charges converted by the voltage-electric charge conversion means are transferred by the charge transfer means. However, since this device uses the mechanism that the constant amount of electric charges is returned as undesired electric charges to the voltage-electric charge conversion means, the amount of the undesired electric charges cannot be controlled in consideration of a change in received light amount.
By the way, it has been known from the past that the photodetector is used for a spatial information detecting device. In the spatial information detecting device, light is projected from a light emission source into a target space, and a reflected light from the target space is received by the photodetector. According to the received light output of the photodetector, spatial information such as a distance to an object in a target space, reflectance of the object, and light transmission in the target space can be detected. When this detecting device is used to detect the spatial information from the target space, in which environment light fluctuates with time, the fluctuation component of the environment light is contained in the received light output of the photodetector. Since this means that an amount of undesired electric charges depending upon the environment light condition, it is impossible to accurately take out the effective electric charges needed to determine the spatial information from the received light output of the photodetector by only separating the constant amount of electric charges as the undesired electric charges, as in the case of the above-mentioned prior art.
Thus, from the viewpoint of stably obtaining a wide dynamic range and achieving an improvement in detection accuracy even under the environment light, which fluctuates with time, the conventional photodetector still has plenty of room for improvement.